The story
Etymology from Wiktionary, CC BY-SA 4.0.
Today it means: A dedicated block usually at the beginning (first block on first track) of a storage medium that holds special data used to start a system. Some systems use a boot block of several physical sectors, while some use only one boot sector. Other manufacturers use the terms boot block and boot sector interchangeably. Full definition →
Etymology from Wiktionary, CC BY-SA 4.0.